Historically, most removable tack down adhesives that fail adhesively from one of the substrates are formulated with styrenic block copolymers containing hydrogenated midblocks, such as styrene/ethylene-butylene/styrene (SEBS) or styrene/ethylene-propylene/styrene (SEPS) block copolymers. These types of formulations have high oil loading to give easy release and are soft, flexible, and have elastomeric characteristics. These adhesives are used to hold in place mailer items or advertisements and are very suitable for non-porous surface applications like plastic cards, films, foils, etc. However, these types of hot melt formulations are not suitable for porous substrates like paper mailer stock due to oil staining. This is mainly due to the high oil concentration of the adhesive composition and/or the composition of or type of paper stock used. In addition, these types of adhesives have higher modulus and are designed to fail adhesively from one of the substrates. However, with paper mailer stock applications, if the adhesive is not applied properly, the higher modulus of the adhesive can cause tearing of the paper when opening the mailer.
U.S. Pat. Nos. 5,912,295 and 6,433,069, both assigned to H. B. Fuller, describe removable grade hot melt pressure sensitive adhesives used in this type of application. Both are directed at adhesives which fail adhesively from one of the substrates. There is no mention of making a product that fails cohesively and instead they state that the “removable hot melt pressure sensitive adhesives are designed to allow separation of substrates at any time after application of one substrate to another without substrate failure or adhesive transfer.” (col. 1, line 32 of '295) There is nothing to suggest the use of a secondary polyolefin polymer which can be used to cause cohesive failure to occur.
U.S. Pat. Nos. 5,741,840 and 6,172,156, both assigned to H. B. Fuller, describe hot melt adhesives which fail cohesively for use in food packaging applications. In this case however, the substrates used are heavy gauge plastic films used for packaging perishable foods. The adhesive bonds strongly to the substrates and fails cohesively when opened. Since the adhesive is pressure sensitive the package can be opened and resealed repeatedly. No mention of staining is made in either of those patents, since the films used would not absorb oil which could cause staining. The hot melts of the present invention also fail cohesively but do not stain porous substrates and are not pressure sensitive adhesives. This requires a completely different formulation approach.
A need exists for a non-pressure sensitive hot melt adhesive which will serve to bond porous substrates together without staining and which will fail cohesively when opened and thereby eliminate tearing of the substrate.